SUMMARY Accumulation of neurofibrillary tangles comprised of Tau, or mutations in the gene encoding this protein, are common in Alzheimer?s disease (AD) and related dementias such as frontotemporal dementia (FTD). Tau contributes to neurodegeneration via two related pathways. First, Tau forms aggregates that are toxic to neurons. Autophagy and lysosomal degradation are of particular interest when considering the removal of Tau aggregates. The idea here is that potentiation of autophagy and lysosomal degradation would rid the cells of aggregates, and thus, alleviate neurotoxicity. Second, at a cellular level, Tau perturbs several key aspects of neuronal function. Studies have shown that mutant Tau can lead to Ca2+ dyshomeostasis, perturbations in neuronal excitability and bioenergetics, and alterations in transcriptional and translational programs. The overarching goal of our exploratory grant application is to bridge these two concepts. We built the aims of this project on the basis of preliminary studies that demonstrate the onset of severe lethality upon expression of human tau or tauR406W in Drosophila glutamatergic neurons. We propose the involvement of two distinct modules of Ca2+ dyshomeostasis working in sequence to impart dysfunction. The first involves elevated inositol trisphosphate (IP3) production and IP3 receptor (IP3R)-mediated ER Ca2+ release. The Ca2+ coming out of the ER is loaded into endolysosomes, and subsequent release of vesicular Ca2+ through the TRPML endolysosomal Ca2+ release channels leads to decline in animal viability. In support of this model, knockdown of genes encoding IP3R or TRPML almost completely prevented the lethality stemming from either Tau or TauR406W. Furthermore, epistasis analyses place TRPML downstream of IP3R in the sequence of events leading to neurotoxicity. In this application, we propose three specific aims to delineate the mechanisms underlying these intriguing findings. In Aim 1, we will determine the cell biological correlates of IP3R- and TRPML-dependent decline of organismal viability. We will test the hypothesis that neuronal cell death imposed by Tau or TauR406W are either delayed or prevented by the genetic attenuation of the IP3R?TRPML axis. In Aim 2, we will test the hypothesis that Ca2+ released by IP3Rs is sequestered into endolysosomes. Thus, we will determine whether IP3R hyperactivation in neurons expressing tau variants leads to an increase in endolysosomal Ca2+ content and elevated TRPML activity. Finally, in Aim 3, we seek to elucidate the mechanisms linking TRPML-mediated endolysosomal Ca2+ release with neurotoxicity. Successful completion of these aims would point to the critical involvement of an IP3R?TRPML axis in neurotoxicity resulting from Tau variants, and motivate future studies designed to evaluate the conservation of these pathways in mouse models of AD and FTD.